621816domain: NAppears in sequencesNumber of Twopins positions.at n=33A005689Number of n-node rooted identity trees with thinning limbs and root outdegree (branching factor) 3.at n=20A245122Expansion of (1 - x - x^3)/((1 - x - x^3)^2 - 4*x^4).at n=19A375279